Modulation of oncogene expression by DNA oligodeoxynucleotides (ODNs) that target messenger RNAs and result in translational inhibition and down-regulation of the corresponding specific target proteins has long been pursued as a therapeutic strategy in cancer. Both chronic and acute leukemia's have been regarded as suitable for this therapeutic approach because of the occurrence of non-random molecular abnormalities that de-regulate gene expression and directly contribute to leukemogenesis. To date mRNA targeted therapies have relied mostly on antisense or reverse complementary DNA ODN-based approaches. When administered in vivo, however, these molecules suffer from a variety of limitations that have recently been partially overcome by modifying the backbone structure of these compounds. Even then, the efficiency of ODNs for target downregulation remains overall low. The recent discovery of cellular endogenous microRNAs (miRs), short RNA sequences that by hybridizing to target RNAs regulate their translation rate, has offered a sound alternative to the antisense DNA oligonucleotides. This concept has been supported by the recent discovery that alterations in the levels of specific miRs are mechanistically relevant to malignant transformation through deregulation of target oncogene or tumor suppressor gene expression. MiRs offer the advantage of being normal counterparts to oncogene or tumor suppressor gene regulation and thereby appear to promote prolonged modulation of the relevant targets and more importantly to target simultaneously several genes involved in pathways regulating cell proliferation, differentiation and survival. These results have recently prompted the design of potential therapeutic applications of synthetically manufactured miRs in cancer. Here, we propose to investigate the clinical applicability of miR-based therapies in-vivo in mouse models where downregulation of specific miRs contributes to malignant transformation and/or aggressive phenotypes of the underlying leukemia. Our group has recently identified relevance of miR-29b in the pathogenesis of epigenetic progression in chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML) through modulation of DNA methyltransferases [DNMT1, DNMT3A and DNMT3B], DNA hypermethylation and gene silencing and miR-181a in tumor suppressor activity by modulating pathways involved in mechanisms of innate immunity, regulating Toll like receptor 4, IL- and miR155 expression in AML cell lines and primary blasts. Therefore, as proof of principle miR-based therapy is an applicable and effective therapeutic strategy. Hence, we propose to perform preclinical in-vivo pharmacokinetic (PK), pharmacodynamic (PD), and therapeutic evaluation of synthetic 2-OMemiR-29b and 2-OMemiR-181a with the goal to validate the pharmacokinetic, pharmacodynamic and therapeutic endpoints, using transgenic (CLL) and xenograft (AML) murine leukemia models. At completion of this project, we anticipate the beginning of initial phase I studies with miR based therapy using miR29b and miR-181 in cancer patients.